btrfs: Remove fs_info from insert_tree_block_ref

This function is always called with a valid transaction so there is no
need to duplicate the fs_info, we can reference it directly from the
trans handle. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>